New approaches for storing liquid hydrogen

Researchers from the Ulsan National Institute of Science and Technology (UNIST), Ewha Womans University, the Institut Laue Langevin and the MLZ report that the modelled storage duration has more than tripled, with liquid hydrogen reaching up to 97% of its theoretical volume capacity.

In the future, storage materials (MOFs, metal-organical frameworks) could help prevent evaporation losses during the transport of liquid hydrogen. © ChatGPT, bearbeitet

In the future, storage materials (MOFs, metal-organical frameworks) could help prevent evaporation losses during the transport of liquid hydrogen. © ChatGPT, bearbeitet

Liquid hydrogen (LH₂) is attractive for long-distance energy transport because of its high volumetric energy density. But even well-insulated tanks cannot completely prevent heat from entering. As the liquid warms, hydrogen evaporates and pressure builds, leading to boil-off losses during storage and transport.

Porous material traps hydrogen

A research team, led by Professor Hyunchul Oh of the Department of Chemistry at UNIST, together with Professor Hoi Ri Moon of Ewha Womans University, and Dr. Jitae T. Park of the Technical University of Munich and Dr. Mónica Jiménez-Ruiz of the Institut Laue–Langevin has investigated whether porous materials could help limit these losses. The team used metal-organic frameworks (MOFs)―highly porous crystalline materials―to capture evaporating hydrogen and delay its release as heat enters the tank.

Hydrogen changes when heated

MOFs contain networks of nanoscale pores that can absorb hydrogen onto their internal surfaces. At cryogenic temperatures, interactions between hydrogen molecules and the pore walls help keep the gas confined, slowing pressure buildup. The approach complements conventional insulation―instead of focusing only on limiting heat from entering the tank, it also changes how hydrogen responds once heat enters.

Less space in the tank?

The key question is whether that benefit comes at the expense of storage capacity. A porous material takes up space that would otherwise hold liquid hydrogen. The researchers therefore compared two MOFs with very different structures―IRMOF-20, a rigid framework with a large pore volume, and MIL-53(AI), a flexible framework whose pores expand and contract as hydrogen is adsorbed.

Dr Jitae Park, pictured here at the MLZ’s PUMA instrument, helped to plan and carry out the measurements. © Wolfgang Filser, TUM

Dr Jitae Park, pictured here at the MLZ’s PUMA instrument, helped to plan and carry out the measurements. © Wolfgang Filser, TUM

Hydrogen packed more densely and more space-efficient

IRMOF-20 offered a particularly favorable balance. When the researchers accounted for the space occupied by the material, the system retained about 97% of the volumetric capacity of neat liquid hydrogen. This was possible because hydrogen confined within the pores packed densely enough to compensate for much of the space taken by the MOF.

The tank remains leak-proof three times as long

The researchers then modeled how the materials would affect boil-off in a transport-scale liquid hydrogen tank. Under the mid-vacuum insulation conditions examined, neat liquid hydrogen was predicted to deplete after about 64 days. With IRMOF-20, that period increased to approximately 221 days – more than three times as long.

Neutrons confirm the theory

Experiments offered clues to why IRMOF-20 performed this way. Adsorption measurements indicated that hydrogen confined within its pores reached an effective density higher than that of bulk liquid hydrogen. Inelastic neutron scattering at the IN1 at the ILL, performed and planned by Jitae Park from the Heinz Maier-Leibnitz Zentrum at TUM and Mónica Jiménez-Ruiz at ILL, also showed restricted molecular rotation inside the pores, providing indirect evidence of strong interactions between hydrogen and the framework. Together, the results suggest that nanoscale confinement can both pack hydrogen densely and stabilize it against release as temperature rises. “Neutrons are particularly well suited to the study of H₂, as hydrogen has a large neutron scattering cross-section and inelastic neutron scattering directly examines molecular motion.”, explains Dr. Jiménez-Ruiz. And Dr. Jitae Park adds: “Inelastic neutron scattering allowed us to directly observe the quantum rotational behavior of hydrogen inside the pores. This microscopic view proved that local environmental confinement creates a dense phase capable of significantly slowing down boil-off losses.”

Greater storage capacity and thermal retention

MIL-53(AI) behaved differently. It retained hydrogen more strongly as temperature increased, but its volumetric capacity was only about 53% of that of liquid hydrogen. The contrast highlights a central design choice for porous materials in cryogenic hydrogen storage―larger pore volume favors capacity, while stronger confinement can improve thermal retention.

A practical test in the tank is still to come

“By considering pore volume, hydrogen density, and desorption behavior together, we found that porous materials could reduce boil-off while preserving much of the storage capacity needed for liquid hydrogen transport,” said Professor Oh. “These calculations represent an idealized upper bound, so further work is needed to determine how closely this performance can be reproduced in practical tank systems.”

Original text: UNIST